Plastic watch casing with plastic crystal and process for joining the crystal to the casing

ABSTRACT

To simplify fitting and sealing the crystal (4) to the casing (2, 6, 8), assembly is effected by means of ultrasonic welding. The casing (2, 6, 8) and the crystal (4) are made of synthetic materials which are thermo-weldable to each other. The mating faces (8a, 4b) of the peripheral wall (8) of the casing and the crystal (4) are formed for reducing the contact area between the two parts to permit thermo-welding by subjecting the assembly to high-frequency mechanical vibrations imparted by a tool (15).

DESCRIPTION

1. Technical Field

The present invention concerns plastic watch casings having plasticcrystals and a process for joining the crystal to the casing. Moreprecisely, the invention is concerned with joining a crystal oftransparent organic material to a plastic watch casing, with the jointbeing sealed and permanent.

2. Background Art

Various processes for sealing the crystal to the casing of a watch havebeen proposed, which make use of the thermoplastic properties ofcrystals of certain organic materials. According to U.S. Pat. No.2,607,082, a watch crystal made of thermoplastic material and providedwith an external groove is pressed into a metal rim on a watch casing.The metal rim has a rib which projects inwardly of the watch casing toengage the groove in the plastic crystal. The assembly of rim andcrystal is then placed on a carrier within coils of an induction heatingapparatus. The metal rim is thus heated, which softens the plasticmaterial of the crystal in the region of the rib on the rim, whereby theplastic material is caused to flow around the rib, producing a sealedjoint. It will be readily appreciated, however, that this process cannotbe used if the watch casing is itself made of a plastic material.

French Patent Application No. 2,491,644 proposes, for the purposes ofsealing a crystal of organic material to a watch casing, that the rimfor mounting the crystal be provided with a groove which is directedinwardly of the rim. The crystal is supported on a shoulder portion ofthe rim and a heated punch member of annular shape is applied to theperiphery of the crystal to cause plastic deformation thereof, therebycausing a part of the plastic material to flow into the groove in therim. While such a process makes it possible to produce a sealed jointbetween the crystal and the rim while avoiding the use of a mechanicaljoining member or any adhesive, the process produces results which arenot generally satisfactory from the aesthetic point of view. In fact,the deformation which is due to the annular punch member being appliedto the crystal occurs at the front face thereof. In addition, suchmethods of fixing the crystal in place involve not inconsiderablemechanical forces in order to cause deformation of the plastic. Suchforces induce in the final article internal stresses which affect theageing qualities of the casing to which the crystal is joined.

DISCLOSURE OF THE INVENTION

In order to remedy such disadvantages, a first object of the presentinvention is to provide a process for joining a plastic crystal to aplastic watch casing, which does not require the application ofsubstantial mechanical forces.

A second object of the invention is to provide a joining process whicheffectively ensures a sealed joint, without preliminary complexmachining operations, or delicate operations to be performed whenassembling the components.

A third object of the present invention is to provide a process formounting the crystal in a casing of synthetic plastic material, whichmakes it possible to enhance the rigidity of the casing, by virtue ofthe presence of the crystal.

To attain such aims, a watch according to the present inventioncomprises a casing and a crystal which are both made of thermoplasticmaterials which can be thermo-welded or heat-sealed together. The matingfaces at the periphery of the crystal and on the casing are preferablymachined to define small surfaces of contact therebetween. Mechanicalenergy, for example in the form of ultrasonic energy, is applied to thecrystal after it has been set in position on the casing. Such energy ispartially transmitted to the casing by way of the small surfaces ofcontact, thereby producing localized heating and partial fusion of thecrystal and the casing at the location of surfaces of contact. Theresult is a fused or welded joint caused by localized migration of thematerials forming the casing and the crystal.

In this way the heating effect is highly localized in the crystal and inthe casing, which gives rise to deformation phenomena which are equallyhighly localized. In addition, the assembly operation is effectivelyperformed by the thermo-welding operation and not by a mechanicalconnection which is produced by deformation, for example, of theperiphery of the crystal. Accordingly, the generation of internalstresses in the crystal and in the casing is minimized. Thethermo-welding and the resultant partial fusion of the case and crystalmaterials ensure a permanent connection and a very good seal between thecrystal and the watch casing. The thermo-welding is accompanied by axialdisplacement of the crystal, which makes it possible to overcomedimensional errors, within certain limits. The invention also concerns aprocess for joining a plastic watch casing to a plastic crystal bythermo-welding the two elements.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will be more clearlyapparent from the following description of a number of embodiments ofthe invention which are given by way of non-limiting example. Thedescription refers to the accompanying drawings in which:

FIG. 1 is an elevation, sectional view of a watch, for illustrating theprocess of this invention;

FIG. 2 is an enlarged, elevation, sectional view of part of theperiphery of the crystal and the periphery of the casing, showing afirst embodiment of the geometry of the mating faces;

FIGS. 3a and 3b are views respectively of part of the periphery of thecrystal and the periphery of the casing, showing a second embodiment ofthe geometry of the crystal and the watch casing;

FIG. 4a shows a second alternative embodiment of the geometry of thecasing and the periphery of the crystal; and

FIGS. 4b and 4c respectively show in greater detail a preferred geometryof the periphery of the crystal, which can be used in the secondalternative embodiment illustrated in FIG. 4a, and the finalconfiguration of the joined casing and crystal, after the thermo-weldingstep.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows in simplified form a watch casing 2 and a crystal 4 whichis not yet secured to the casing and which serves to close the casing.In the embodiment illustrated, the casing 2 is a monobloc casing inwhich the bottom 6 and the peripheral wall 8 of the casing are made fromone and the same piece. As is well known, the casing contains a movementwhich is diagrammatically indicated by the rectangle 10 from whichproject the spindles 12 and 14 for the hands, extending through a dial16. According to the invention, the casing 2 is made of thermoplasticmaterial. In such a case, the internal surfaces of the casing, in mostinstances, comprise a certain number of machined or molded features usedfor fixing or positioning certain elements of the movement. As is wellknown, the movement 10 essentially comprises a stepping motor, a geartrain for driving the spindles 12 and 14, an electrical circuit and atime base for controlling the motor, and finally a battery for supplyingpower to the motor and the various electronic components. It will beseen that the illustrated embodiment is an analog display watch ofelectronic type. It will be clearly apparent to the man skilled in theart that the invention could equally well be applied to a mechanicalanalog-display watch or a digital-display electronic watch.

The casing 2 and the crystal 4 are made of thermoplastic materials whichcan be thermo-welded together. The casing 2 may be formed for examplefrom one of the materials commonly referred to as ABS, ASA or SAN, whichrespectively correspond to an acryl butadiene styrene, an acryl styreneacrylester and a styrene acryl nitrile. In such cases, the crystal 4 ismade of a transparent acrylic PMMA (polymethylmethacrylate), such asthose sold under the trademark Plexiglass. The casing 2 and the crystal4 may also be made from polycarbonates, the polycarbonate forming thecrystal obviously being transparent. It will be appreciated that it ispossible to use other pairs of materials, provided that they can bethermo-welded together.

It should also be noted that the peripheral wall 8 and the periphery 4aof the crystal 4 have mating faces 8a and 4b. At the time of assembly,the faces 4b and 8a of the crystal and the casing are disposed againsteach other. According to the invention, and as will be described ingreater detail hereinafter, at least one of the faces 4b and 8a ismachined to provide a small surface of contact therebetween and therebypermit the two parts to be effectively welded together and produce asealed joint. In order to effect thermo-welding the two parts,sufficient pressure is applied to the upper face 4d of the crystal 4 toensure intimate contact between the mating faces 4b and 8a. At the sametime, energy is transmitted to the peripheral side wall 8 at thelocation of the face 8a, by virtue of the close contact between the twosurfaces. The combination of the pressure applied to the crystal at asmall surface of contact and the application of energy produces highlylocalized heating and fusion of a part of the mating faces, thusproducing a real weld between the two parts. That is, not only do thetwo parts experience highly localized heating, but also thethermo-welding operation produces interpenetration by fusion of thematerials forming the two parts in question.

Preferably, the thermo-welding operation is effected by ultrasonicmeans. Using a tool 15 known in the art as a sonotrode, a mechanicalvibration of an amplitude in the range of 3 to 50 μm is transmitted tothe crystal 4, at a high frequency of the order of 40 kHz. The whole ofthe crystal begins to oscillate synchronously with the sonotrode. Byproviding a zone of lower mechanical strength in the contact zone,considerably higher stresses are generated at the contact zone than inthe remainder of the system. The first zone to begin to go into acondition of fusion is that which absorbs the greatest mechanicalforces; so that, the fusion effect does in fact occur in the contactzone. As will be more clearly apparent from the description hereinafter,the geometries of the mating faces 8a and 4b are such as to provide alocal zone of lower mechanical strength, which is the first zone tobegin to fuse.

FIG. 2 shows one embodiment of a suitable geometry for the mating facesof the crystal and the peripheral side wall of the watch casing. In FIG.2, the two parts are shown in a position before the thermo-weldingoperation. As the crystal and casing are machined to be rotationallysymmetrical about the axis of the watch, at least when the watch iscircular, only their cross-sectional geometries will be describedherein. Starting from the region which is the most inwardly disposedregion of the watch and from the inside surface of peripheral wall 8,the illustrated construction has axially and radially spaced, radiallyextending interior portions 20, 22, 24 which are parallel to the planeof the watch; a radially extending external portion 26 at the upper edgeof peripheral wall 8; and axially extending interior portions 28, 30, 32which are parallel to the axis of the watch. The portions 20, 22, 24 and28, 30, 32 thus define three steps, going from the center of the watchtoward the outside thereof and the upper face of the casing. Similarly,the assembly face 4b at the periphery of the crystal comprises threeaxially and radially spaced, radially extending portions 34, 36, 38which are parallel to the plane of the watch casing. Portions 34, 36, 38are connected by a radially outwardly extending, inclined portion 40 andby two axially extending portions 42, 44 which are parallel to the axisof the watch. As shown, portion 44 extends from portion 38 to theoutside face 4d of the crystal.

Portions 20, 28 define an annular recess for receiving the periphery ofthe dial 16. The portions 22 and 30 define an annular recess into whichprojects the frustoconical part of the crystal defined by the inclinedportion 40, so as to center the crystal on the casing during thethermo-welding operation. The radially extending portion 36 of theperiphery of the crystal, which is substantially narrower than theradially extending portion 24 of the peripheral wall 8 disposed oppositethereto, defines a zone l involved in the thermo-welding operation, zonel being of reduced mechanical strength. A radial clearance is providedbetween the axially extending portion 32 of the peripheral wall 8 andthe axially extending portion 44 of the crystal. By way of example, inthe case of a watch whose outside diameter is of the order of 31 mm, thedimension l involved in the thermo-welding operation, that is, the widthof the zone of contact of the portions 24 and 36, is about 0.25 mm. Itwill be seen therefore that the surfaces which are actually in contactare of very small area.

In addition, before the thermo-welding, an axial clearance e is providedbetween radially extending portions 22 and 34. For example, theclearance e is about 0.2 mm. During the thermo-welding operation, underthe effect of the pressure applied, radially extending portions 24 and36 are brought into intimate contact. For the reasons indicated above,this causes partial fusion of portion 36 of the crystal and portion 24of peripheral wall 8, and causes the crystal to be pushed inward underthe effect of the applied pressure until radially extending portions 22and 34 come into contact. As already referred to above, thethermo-welding preferably is achieved by ultrasonic energy which isapplied to the crystal by means of a tool 15 such as a sonotrode.

The purpose of the radial clearance between axially extending portions32 and 44 is to limit the contact surfaces between the crystal and theperipheral wall 8 to those surfaces where the thermo-welding is actuallyto occur, that is, to the annular contact surface defined by radiallyextending portions 36 and 24. This arrangement avoids wasting energysince all the ultrasonic energy is concentrated in the narrow annularcontact surface of radially extending portions 36 and 24. The projectingor overlapping part of the radially extending portion 34 of the crystal,by covering the periphery of the dial 16, ensures that the dial is heldin position in the recess defined by radially extending portion 20 andaxially extending portion 28. With regard to the inward movement e ofthe crystal relative to the casing during thermo-welding, it is possibleto produce a sealed weld between the crystal and the casing, even ifthere are defects in respect of co-planarity between radially extendingportions 24 and 36, provided of course that such defects do not exceedthe value e. Significantly, the only effect of the pressure applied tothe crystal is to cause the crystal to be pushed inward as local fusionprogresses between the crystal and the peripheral wall 8. The forces aretherefore low and the internal stresses resulting therefrom are alsogreatly reduced. The foregoing description relates to the case of arotationally symmetrical crystal. The man skilled in the art willreadily appreciate that the process may be applied irrespective of thecomplexity of the contour of the glass.

FIGS. 3a and 3b show an alternative form for the peripheral geometriesof the crystal and the casing. FIG. 3a shows the geometry of theassembly face of the periphery 4a of the crystal. The machined portionscomprise a radially extending contact face 50 which is parallel to theplane of the casing, and from which projects an annular axiallyextending rib 52. The lower end of the rib 52 terminates in a taperedportion 56 having a sharp edge 56a. As can be seen from FIG. 3b, thegeometry of the assembly face of the peripheral wall 8 comprises a firstradially extending recess or face portion 58 for receiving the dial 16,and a second axially extending face portion 60 for receiving theperiphery of the glass. Portion 60 terminates at an axially extending,annular groove 62 having a flat bottom 62a on which the sharp edge 56arests prior to thermo-welding. During thermo-welding, the sharp edge 56ais in contact with the bottom 62a of the groove 62, thereby forming azone of reduced mechanical strength. The axial length of the rib 52 isgreater than the axial depth of the groove 62, to leave the clearance ewhen the parts are set in position before the thermo-welding operation.Thus, the energy applied to the crystal causes heating and local fusionof the edge 56a and the peripheral wall 8 in the vicinity of the bottom62a, which results in a welding effect, at the location of the sharpedge 56a of the rib. In this operation, the crystal is also pushedinwardly relative to the casing, with such inward movement being limitedby the abutment face 50 of the crystal, which also comes up against theperiphery of the dial 16 in order to fix it in position.

The embodiments illustrated in FIGS. 2 and 3 leave a part of theperipheral wall 8 visible on the front face of the watch. The embodimentillustrated in FIGS. 4a, b, c corresponds to the situation where thecrystal is to cover the whole of the front face of the peripheral wall8. As can be seen from FIG. 4b, the geometry of the periphery of thecrystal comprises a radially extending support or contact face 70 fromwhich projects an annular axially extending rib 72 which may beidentical to that shown in FIG. 3a and has a tapered sharp-edged portion74. As shown in FIG. 4a, the geometry of the assembly face of theperipheral wall 8 comprise a radially extending recess or face 78 forreceiving the dial 16 and a radially extending support surface 80 havingtherein an axially extending, annular groove 82 with a flat bottom 82a.In the thermo-welding operation, the ultrasonic vibration producespartial fusion of the sharp-edged portion 74 and the bottom 82a of thegroove, causing the two parts to be welded together and causing thewhole crystal to be moved inwardly with respect to the casing, under theeffect of the pressure applied thereto.

FIG. 4c shows the two parts after the thermo-welding operation. Theabutment face 70 has come into contact with assembly face 80 of theperipheral wall 8. The thermo-welding has caused the pointed edgeportion 74 to disappear and has resulted in a fused region indicated at84, in which the material is formed by interpenetration of the materialsrespectively forming the peripheral wall 8 and the crystal.

It will be apparent from the foregoing description that the inventionactually makes it possible to produce a sealed, irremovable mounting forthe crystal on the watch casing, by means of simple operations which donot significantly affect the mechanical strength of the two parts to beassembled. It will also be appreciated that, having regard to the factthat the crystal and the casing are joined together by thermo-welding,the two parts behave from the mechanical point of view as if they formedjust one piece. That results in a substantial improvement in therigidity of the whole watch casing. This is a highly desirable resultinsofar as, in most cases, many machined or molded features are formedin the bottom of the casing, for housing the various components of thewatch. The rigid interconnection of the crystal and casing avoids theneed to provide additional components for reinforcing the casing.

We claim:
 1. A process of joining a crystal to a watch casing,comprising the steps of:providing a watch casing of a thermoplasticsynthetic material, said casing having an assembly face and an abutmentface; providing a crystal of a transparent synthetic material which isthermo-weldable to the material forming said casing, said crystal havinga periphery provided with an assembly face and an abutment face;providing a dial; placing said dial and said crystal on said casing in amutual superimposed relationship over said casing such that saidassembly faces and said abutment faces are respectively at leastpartially facing each other, with the dial located between said crystaland said casing; exerting a pressure on said crystal to bring at least aportion of each of the assembly faces into contact with each other whiletransmitting thermo-welding energy to the crystal and the casing tocause localized heating and fusion of the materials forming the portionsof the crystal and the casing which are in contact in order to weld saidcrystal to said casing, said assembly faces providing sufficiently smallsurfaces of contact to cause said localized heating and fusion in thepresence of said thermo-welding energy; and, maintaining said pressureand said energy until said dial is locked between said abutment faces soas to mount said dial directly on said casing beneath said crystal.
 2. Aprocess according to claim 1, wherein said energy is supplied in theform of high-frequency mechanical vibrations.
 3. A process according toclaim 2, further comprising the step of providing the assembly faceswith geometries defining said portions which are in contact, saidportions in contact having a substantially reduced surface area withrespect to that of the assembly faces.
 4. A process according to claim3, wherein the assembly face of the crystal comprises a rib whichprojects from said face, said rib terminating with a pointed-edgeportion, and the assembly face of the casing comprises a groove forpermitting engagement of said rib thereinto, said groove having asubstantially flat bottom.
 5. A process according to claim 4, whereinthe casing and the crystal are made of polycarbonates.
 6. A processaccording to claim 3, wherein the material forming said casing isselected from the group consisting acryl butadiene styrene (ABS), acrylstyrene acryl-ester (ASA) and styrene acryl nitrile (SAN), and thematerial forming the crystal is a transparent acrylic resin (PMMA).
 7. Aprocess according to claim 3, wherein the casing and the crystal aremade of polycarbonates.
 8. A process according to claim 2, wherein thematerial forming said casing is selected from the group consisting acrylbutadiene styrene (ABS), acryl styrene acryl-ester (ASA) and styreneacryl nitrile (SAN), and the material forming the crystal is atransparent acrylic resin (PMMA).
 9. A process according to claim 2,wherein the casing and the crystal are made of polycarbonates.
 10. Aprocess according to claim 1, further comprising the step of providingthe assembly faces with geometries defining said portions which are incontact, said portions in contact having a substantially reduced surfacearea with respect to that of the assembly faces.
 11. A process accordingto claim 10, wherein the assembly face of the crystal comprises a ribwhich projects from said face, said rib terminating with a pointed-edgeportion, and the assembly face of the casing comprises a groove forpermitting engagement of said rib thereinto, said groove having asubstantially flat bottom.
 12. A process according to claim 11, whereinthe casing and the crystal are made of polycarbonates.
 13. A processaccording to claim 10, wherein the material forming said casing isselected from the group consisting acryl butadiene styrene (ABS), acrylstyrene acryl-ester (ASA) and styrene acryl nitrile (SAN), and thematerial forming the crystal is a transparent acrylic resin (PMMA). 14.A process according to claim 10, wherein the casing and the crystal aremade of polycarbonates.
 15. A process according to claim 1, wherein thematerial forming said casing is selected from the group consisting acrylbutadiene styrene (ABS), acryl styrene acryl-ester (ASA) and styreneacryl nitrile (SAN), and the material forming the crystal is atransparent acrylic resin (PMMA).
 16. A process according to claim 1,wherein the casing and the crystal are made of polycarbonates.
 17. Awatch according to claim 1, wherein said casing is made of a materialselected from the group comprising acryl butadiene styrene (ABS), acrylstyrene acryl-ester (ASA) and styrene acryl nitrile (SAN), and thecrystal is made of a transparent acrylic resin (PMMA).
 18. A watchaccording to claim 1, wherein said casing and said crystal are made withpolycarbonates.
 19. A process according to claim 4, wherein saidassembly face and said abutment face of said crystal are defined in acommon plane.
 20. A process according to claim 11, wherein said assemblyface and said abutment face of said crystal are defined in a commonplane.
 21. A process according to claim 1, wherein said assembly faceand said abutment face of said crystal are joined to each other by aslanting surface portion allowing the crystal to be centered withrespect to said casing when placing said crystal on said casing.
 22. Awatch comprising a crystal joined to a watch casing with a dial mounteddirectly on said casing beneath said crystal in accordance with theprocess of claim
 1. 23. A watch according to claim 22, wherein theassembly face of said crystal comprises a rib which projects from saidface, and the assembly face of the casing comprises a groove receivingsaid rib, said localized fusion providing a fused mixture located at thefree end portion of said rib and at the bottom of said groove.
 24. Awatch according to claim 23, wherein said abutment face and saidassembly face of said crystal are defined in a same plane.